1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
The substrate may be subjected to a variety of processes before the exposure process. For example, the substrate will generally be treated with resist before exposure. Also, prior to exposure, the substrate may be subjected to cleaning, etching, ion implantation (e.g., doping), metalization, oxidation, chemo-mechanical polishing, priming, resist coating, soft bake processes, and/or measurement processes. The substrate may also be subjected to a variety of post-exposure processes, for example, post-exposure bake (PEB), development, hard bake, etching, ion implantation (e.g., doping), metalization, oxidation, chemo-mechanical polishing, cleaning, and/or measurement processes. Furthermore, typically several layers are required. Therefore, the entire processing procedure or variant thereof, is repeated for each layer.
The pre-exposure and post-exposure processes are performed by stations or modules designed for their respective purposes. The substrate is subjected to these processing modules, as well as the lithographic exposure apparatus, in a pre-defined sequence. In order for devices formed on a substrate to function correctly, it is necessary for the features of those devices formed on the substrate to be within a given tolerance of a specified size. It is therefore standard to specify key attributes, which are collectively referred to as a critical dimension (CD) of the exposed pattern, in order to characterize the features and the profile of the pattern and establish a bench mark level of quality and uniformity. The CD metric may include, for example, a gap between features, X and/or Y diameter of holes and/or posts, ellipticity of holes and/or posts, an area of a feature, a feature side wall angle, a width at the top of a feature, a width at the middle of a feature, a width at the bottom a feature, line edge roughness, or the like.
The size of a feature formed on a substrate is, in general, determined by the areas of resist on the substrate that receive a threshold dose of radiation in the exposure process. This threshold dose is affected by the type of resist used, the processing conditions before exposure, and the processing conditions after the exposure. Therefore, variations in the pre-exposure and post-exposure conditions affect the size of features, and hence, the critical dimension uniformity (CDU), potentially compromising the quality of the exposed pattern. If the CDU is not sufficient, a substrate may be rejected, reducing the yield.
Therefore, what is needed is lithographic apparatus and method that can be used to expose patterns on a substrate with improved CDU.